Effect of enhancement additives on the absorption of water vapor by aqueous lithium bromide.

摘要

The objective of this study was to obtain a complete understanding of the enhancement mechanism of heat/mass transfer enhancement additives in aqueous lithium bromide (LiBr) systems. This goal was achieved by conducting four sets of experiments aimed at investigating various aspects of the enhancement mechanism. A new explanation of the enhancement, termed the vapor surfactant theory, resulted from this study and is presented in detail.; In a LiBr absorption system, water vapor is absorbed into falling films of aqueous LiBr. One bottleneck for absorption is mass transfer on the liquid side and this is addressed in commercial equipment using a mass transfer enhancement additive such as 2-ethyl-1-hexanol. The vapor surfactant theory holds that the enhancement additives are surfactants delivered to the surface of the film from the vapor and that they produce enhancement by generating intense secondary flows on the film. The secondary flows, driven by surface tension gradients (Marangoni convection), augment the transfer process by providing mixing of the film. The primary variable in the enhancement is the surface concentration distribution of the surfactant additive since the surface tension gradients arise due to non-uniformities in the surface concentration. The vapor surfactant theory is found to explain all observations from both the current study and the literature.; The study included investigations in the following experimental configurations: (1) absorption of water vapor into aqueous LiBr with and without additive, (2) condensation of water vapor with and without additive, (3) surface tension of aqueous LiBr and water with and without additive, and (4) static pool experiments without significant heat/mass transfer.; Flow visualization of the additive induced secondary flows in absorption and condensation was a primary method used in the study.; Surface tension measurements of aqueous LiBr and pure water with additives show that the surface tension for both decrease and reach a plateau value with increasing additive concentration. Additive vapor was identified as the primary variable that influences the surface tension of aqueous LiBr.; The study also resulted in identifying two new additives which result in heat transfer coefficients comparable to that of 2-ethyl-1-hexanol.